Analog Input Timing Engine

The following figure summarizes all of the timing options provided by the analog input timing engine.

The USB-6451 uses AI Sample Clock (ai/SampleClock) to perform simultaneous sampling on all active analog channels. Since there is one ADC per channel, AI Sample Clock controls the sample period on all the channels in the task. The USB-6451 also uses AI Sample Clock (ai/SampleClock) and AI Convert Clock (ai/ConvertClock) to perform interval sampling in dual-channel scan mode.

As the following figure shows, AI Sample Clock controls the sample period, which is determined by the following equation:

AI Convert Clock controls the Convert Period, which is determined by the following equation:

An acquisition with post-trigger data allows you to view data that is acquired after a trigger event is received. A typical post-trigger DAQ sequence is shown in the following figure. The sample counter is loaded with the specified number of post-trigger samples, in this example, five. The value decrements with each pulse on AI Sample Clock, until all desired samples have been acquired.

An acquisition with pre-trigger data allows you to view data that is acquired before the trigger of interest, in addition to data acquired after the trigger. The following figure shows a typical pre-trigger DAQ sequence. The AI Start Trigger signal (ai/StartTrigger) can be either a hardware or software signal. If AI Start Trigger is set up to be a software start trigger, an output pulse appears on the ai/StartTrigger line when the acquisition begins. When the AI Start Trigger pulse occurs, the sample counter is loaded with the number of pre-trigger samples, in this example, four. The value decrements with each pulse on AI Sample Clock. The sample counter is then loaded with the number of post-trigger samples, in this example, three.